In the early days of assembly of electrical circuits, wires were attached to terminal points by simple screws or screws and nuts. In the next "Radio" generation, wire connections were made by threading the same through solder lugs for ultimate hard soldering later on. The soldering processes themselves evolved from simple hand soldering to "dip" type soldering wherein a multiple number of connections were all soldered simultaneously. In either situation, the wires have their ends stripped and are placed into physical contacts with terminals formed by perforations; for example, holes in a printed circuit board. Normal continuity existed even though the particular wire was not soldered to the terminal point. For example, the wire and might be bent through a performation or opening in a printed circuit board and thus maintain physical contact with the proper terminal even though "loose" in the connection. Similarly, the wire ends might be wrapped about a terminal post thereby effecting physical and electrical contact therewith even though not soldered.
After the various wire ends are physically placed in physical contact with the terminals, the routine soldering operation is carried out either by hand or in some instances by automatic soldering equipment.
After the routine soldering operation, the normal quality control testing of the circuitry is carried out. Since this testing involves continuity and conduction tests of various terminal points, an unsoldered terminal connection will not be detected and as a result a piece of equipment may be shipped out with overlooked solder joints.
Because of continuity and conduction of the wire ends with the terminal points even though not soldered, the equipment may operate satisfactorily even for several months. Ultimately, however, the shiny wire core end will tend to corrode or oxidize with the result of a poor connection and possible arcing which latter phenomenon can be very detrimental to modern solid-state components.
While the various terminal points can be visually inspected to make sure that proper soldering has been carried out, such visual inspection is subject to human error and thus there still exists the problem of defective equipment being purchased by the ultimate consumer.